1. Technical Field
The present invention relates generally to oscillators, and more particularly to the type of oscillator utilized in high frequency power converters where initial and final frequencies are required to be well defined.
2. Related Art
Cellular wireless communication systems support wireless communication services in many populated areas of the world. While cellular wireless communication systems were initially constructed to service voice communications, they are now called upon to support data communications as well. The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, cellular wireless users now demand that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. The demand for wireless communication system data communications will only increase with time. Thus, cellular wireless communication systems are currently being created/modified to service these burgeoning data communication demands.
Cellular wireless networks include a “network infrastructure” that wirelessly communicates with wireless terminals within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC). Each BSC also typically directly or indirectly couples to the Internet.
In operation, each base station communicates with a plurality of wireless terminals operating in its cell/sectors. A BSC coupled to the base station routes voice communications between the MSC and a serving base station. The MSC routes voice communications to another MSC or to the PSTN. Typically, BSCs route data communications between a servicing base station and a packet data network that may include or couple to the Internet. Transmissions from base stations to wireless terminals are referred to as “forward link” transmissions while transmissions from wireless terminals to base stations are referred to as “reverse link” transmissions. The volume of data transmitted on the forward link typically exceeds the volume of data transmitted on the reverse link. Such is the case because data users typically issue commands to request data from data sources, e.g., web servers, and the web servers provide the data to the wireless terminals.
To conserve power, the wireless terminal may sleep when not actively communicating with a servicing base station. However, to ensure no communications are missed, the wireless terminal awakens periodically to receive a page burst that indicates if the wireless terminal must service a communication from the servicing base station. Various other electronic devices may enter a sleep mode as well in order to conserve power. To realize this advantage, the timing associated with the sleep mode should be accurately controlled in order to allow the wireless telephone to awaken at predetermined intervals to check for received messages or pages. Thus, it is important to have an accurate low power oscillator for timing when to awaken from or enter into the sleep mode and effectively conserve power.
One such low power oscillator is a Schmitt trigger RC oscillator. FIG. 1 depicts a general Schmitt trigger RC oscillator 10. Schmitt trigger RC oscillator 10 includes an operational amplifier 12 that receives a first voltage input or threshold voltage input VP and a second voltage input VN. Operational amplifier 12 generates an output voltage, VOUT, equal to a first output voltage, VDD, when VP is greater than VN; or a second output voltage, such as ground, when VP is less than VN. A resistive capacitive (RC) network 14 couples to the output of operational amplifier 12 and supplies VN to operational amplifier 12. Additionally a voltage divider 16 also couples to the output of operational amplifier 12. As shown here, VP is supplied from the voltage divider 16, as the voltage seen at the node between resistors R1 and R2.
When VP>VN, VOUT goes to VDD and begins to charge capacitor CX. This increases the voltage VN. During this charging period, VP1=VREF+R1/(R1+R2)*(VDD−VREF). When VN exceeds the switching point VP, VOUT goes to ground and begins to discharge capacitor CX. This decreases the voltage VN. During this discharging period, VP2=R2/(R1+R2)*VREF. Then, the switching point defined by VP is a function of VDD and VREF. During Charging Period, VN may be defined as
      V    N    =                              V          DD                ⁡                  (                      1            -                          ⅇ                                                -                                      t                    1                                                                                        R                                          X                      ⁢                                                                                                                            ⁢                                      C                    X                                                                                )                    +                        V                      P            ⁢                                                  ⁢            2                          ⁡                  (                      ⅇ                                          -                                  t                  1                                                                              R                                      X                    ⁢                                                                                                                ⁢                                  C                  x                                                              )                      =                  V                  P          ⁢                                          ⁢          1                    .      During the discharging period, VN may be defined as
      V    N    =                    V                  P          ⁢                                          ⁢          1                    *              ⅇ                              -                          (                                                t                  2                                -                                  t                  1                                            )                                                          R              X                        ⁢                          C              X                                            =                  V                  P          ⁢                                          ⁢          2                    .      Solving these two equations, where for example R1=R2, yields an expression for the period of the oscillator be defined
      as    ⁢                  ⁢          t      2        =            -              R        X              *          C      X        *                  ln        (                                  ⁢                                                            1                2                            ⁢                              (                                                      V                    DD                                    -                                      V                    REF                                                  )                                                                    V                DD                            -                                                1                  2                                ⁢                                  V                  REF                                                              *                                    V              REF                                                      V                DD                            +                              V                REF                                                    )            .      Thus, the frequency of the oscillator may be defined as
      1          t      2        ,which is a function (RX,CX,VDD, VREF).
The output of the operational amplifier may be a continuous square wave as shown in FIG. 2. The frequency of this square way depends on the values of R and C and the threshold points of the Schmitt trigger. The Schmitt trigger RC oscillator circuit may be easily incorporated within an integrated circuit (IC). However, it should be noted that the frequency stability is lacking as the frequency is dependent on the input voltage VDD and VREF for the reasons shown above. As the input voltage can vary as much as +/−10 percent, the frequency may also vary +/−10 percent. This level of variation makes the Schmitt trigger oscillator unacceptable as an accurate timing source for determining when to awaken from or enter into the sleep mode and effectively conserve power.